We added theming support back to the Help Viewer in 15.3. One thing that is no longer possible is to keep VS and the Help Viewer themes in sync due to changes in VS in terms of how we store and handle settings. That being said, in the Help Viewer you can go to the options dialog and manually select your desired theme, then close and relaunch the Help Viewer to apply the theme change. Let me know if that works for you!

15.3's version of the Help Viewer should be 2.3.26711.1. If your Help Viewer is a different version after updating could you share your setup logs to see if an error occurred during the update? To collect the setup logs, download and run the collection tool - https://aka.ms/vscollect - and share the log file created in %TEMP%\vslogs.zip. You can upload this file by replying and inserting the files using the insert (attach) button. More instruction on how to run the tool is available on the download page.